330: ' METALLURGY OF IROST AND STEEL.. ' • ' "
"In- the same manner the. elongation in the cylindrical portioni may he calculated for all .the different lengths given in Table; Xyi-0. The results are as follows,, in per cent.::
4"=24.79; 6"=24.54; 8"=24.40; 10"=24.34; 12"=23.96; 14"=23.S2.
There is a decrease in elongation with an increase, in length, and. the relation is so regular that it is probably due to something be*'-sides experimental error. If the necking be assumed to take place within a length of only one inch, instead of two inches, the calcu--lated percentage of elongation will be a little more uniform, but thev improvement is so .slight, even with this extreme hypothesis, that some other cause is.shown to be at work.
I believe that the true explanation is in the fact, which was called to my attention by Mr. W. E. Webster, that the breaking speed varies with each length. The speed of the machine was the same in every case, tint'a constant speed of the grips does not mean a constant rate oi distortion in the,bar. In the case of the 2-inch piece, the stretch was 47.43 per cent., indicating a linear extension of 0.95 inches;! in the case of the 14-inch piece the stretch was 26.76 per cent., indicating an extension of 3.75 inches. The rate of distortion, therefore, was four times as great in the 2-inch test as in the 14-inch bar, and this condition would give a higher elongation with each decrease in length, as shown in Section XYIm. Owing to this complication it is impossible to deduce a theoretically accii-rate .answer from the foregoing data, but in a three-quarter inch round bar of infinite length, of the steel shown in Table XVI-0, the elongation would be about 24 per cent.
• SEC. XVIk.—Tests on eye-bars.—Through the courtesy of The Union Bridge Company, of Athens, Pa., I have had 'access to its records of eye-bar tests, and have classified them to determine the influence of width, thickness and length upon the physical properties. All bars which showed 100 per cent, crystalline fracture, and pieces of miscellaneous lengths when there were less than three bars of the same steel in the group, were omitted. A few pieces were discarded when the elongation in 12 inches was the same as in the full length, for this indicates either a clerical'error or that" fracture took place in the eye. After these eliminations only three works were represented, two of them by both open-hearth and Bessemer steel. The records are given-in Table, XVI-Q, and show